1. Field of the Invention
The present invention relates to a mask material composition, a method of forming an impurity diffusion layer, and a solar battery.
2. Description of the Related Art
In the manufacture of solar batteries, formation of, for example, an N-type or P-type impurity diffusion layer into a semiconductor substrate has conventionally been performed as follows: patterning of a diffusing agent containing an N-type or P-type impurity diffusing component is performed on the surface of a semiconductor substrate; and the N-type or P-type impurity diffusing component is diffused from the patterned diffusing agent to form an N-type or P-type impurity diffusion layer. Specifically, a thermally-oxidized film is first formed on the surface of a semiconductor substrate, and then a resist with a predetermined pattern is laminated on the thermally-oxidized film by a photolithography method. By using the resist as a mask, part of the thermally-oxidized film, which is not masked with the resist, is etched with an acid or alkali and then the resist is peeled off to form a mask of the thermally-oxidized film. Subsequently, a diffusing agent containing an N-type or P-type impurity diffusing component is coated thereon to form a diffusion film on the part where the mask is open. Thereafter, an N-type or P-type impurity diffusion layer is formed by diffusing the impurity diffusing component in the diffusion film at high temperature.
With respect to such manufacture of solar batteries, Japanese Patent Application Publication 2007-49079 has disclosed a masking paste to be used as a diffusion control mask. By using such a masking paste, fine patterning in an impurity diffusion region can be easily formed without using a complicated photolithographic technique, etc., thereby, allowing for solar batteries to be manufactured at low cost.
Mostly, mirror-like finishing is not performed on the surface of a semiconductor substrate to be used in a solar battery. Also, a texture portion having a fine concave-convex structure is mostly formed on the surface of a semiconductor substrate to be used in a solar battery to prevent reflection of light on the surface thereof. Accordingly, when a mask is formed on the surface of such a semiconductor substrate, a mask material sometimes accumulates in the concave portions, thereby causing the film thickness of the mask to be large in the concave portions. In the conventional mask materials, there has been the fear that cracking may occur in the portions where the film thickness of the mask is large, and hence the diffusion barrier performance of the mask material may be deteriorated. Also, in the conventional mask materials, the molecular weight of the resin of which a mask material consists may increase due to a change over time. When the molecular weight of the resin increases, the properties of the mask material, such as viscosity, change, and accordingly the coating stability thereof is sometimes deteriorated.